Pseudohypoparathyroidism (PHP) type 1 is an autosomal dominant disorder characterized by biochemical hypoparathyroidism due to resistance of target tissues to parathyroid hormone (PTH). Two fundamentally different forms of PHP type 1 have been described. In PHP type 1a, mutations in the GNAS1 gene lead to reduced expression or activity of the alpha subunit of the G protein (Gs alpha) that couples heptahelical receptors for PTH and many other hormones to activation of adenylyl cyclase. Accordingly, widespread deficiency of Gs alpha is associated with resistance not only to PTH but also to other hormones that act by stimulating adenylyl cyclase. By contrast, Gs alpha is normal in PHP type 1b, and hormone resistance is limited to PTH target tissues. These observations had suggested that the defect in PHP type 1b was the type 1 PTH receptor, but molecular analysis of this gene has not revealed mutations. The goal of this project is to identify and characterize the gene for PHP type 1b. We have used linkage analysis to assign the locus for PHP type 1b (designated PHP1b) to 10q26. This region contains a candidate gene that encodes the type 5 G protein-coupled receptor kinase (GRK 5), a protein kinase that can phosphorylate and desensitize the type 1 PTH receptor. To determine the PHP1b gene, a positional cloning strategy will be used involving the following approaches: (1) Further refine the PHP1b locus first by haplotype analysis of three additional PHP type 1b families we have ascertained, and subsequently by further analysis of all families using additional polymorphic markers mapped to this region. (2) Screen publicly available YAC clones and assemble a linear contig spanning the locus, and increase the resolution of the physical map. (3) Identify coding sequences in this region by review of publicly available EST maps, screening cDNA libraries by GeneTrapper methodology, and by exon-trapping. (4) Characterize transcripts through evaluation of homology to other known genes, patterns of tissue-specific expression, and cross-species conservation. (5) Perform mutational analysis of genomic DNA samples from familial and sporadic cases. Once the PHP1b gene is identified, the long-range goal will be to characterize the function of the gene product, to examine genotype-phenotype correlations, and to understand its role in regulating expression or action of the PTH receptor in health and disease.